Silicone based release coatings are useful in applications where relatively non-adhesive surfaces are required. Single sided liners, for example, backing papers for pressure sensitive adhesive labels, are usually adapted to temporarily retain the labels without affecting the adhesive properties of the labels. Double sided liners for example, interleaving papers for double sided and transfer tapes, are utilised to ensure the protection and desired unwind characteristics of a double sided self-adhesive tape or adhesive film.
A substrate, for example a single sided liner, is coated by applying a silicone based release coating composition onto the substrate and subsequently curing the composition, by, for example, thermally initiated hydrosilylation. The basic constituents of silicone based release coating compositions which are cured by hydrosilylation are an alkenyl functional polydiorganosiloxane, typically a linear polymer with terminal alkenyl groups, a polyorganohydrogensiloxane cross-linking agent, designed to cross-link the alkenyl polydiorganosiloxane and a catalyst for the cross-linking reaction, usually a complex or compound of a group VIII metal (platinum group metal) such as platinum, ruthenium, rhodium, palladium, osmium or indium.
Improvements in the performance of release coatings are continuously being sought with respect to, for example, ease of cure, i.e. the decrease in cure times at relatively low temperatures, release performance and anchorage of coatings to various substrates, including polypropylene, polyethylene and polyester as well as paper.
EP-A-1070734 describes a release coating composition comprising a branched siloxane consisting of one or more Q units of the formula (SiO4/2), from 15 to 995 D units of the formula Rb2SiO2/2, and M units of the formula RaRb2SiO1/2, at least three Ra substituents in the branched siloxane being alkenyl or alkynyl. Release coatings based on such Q-branched polymers both cure at relatively low temperatures, for example at 80-150° C., particularly 100-120° C., and have improved long term anchorage properties. As the cure is good at low temperatures there is minimal transfer of silicones to an adhesive such as the adhesive on a label, which in turn provides the benefit that the strength of the adhesive is maintained. The branched siloxane can be prepared by mixing a compound having the general formula (SiO4/2)(RaRb2SiO1/2)4 with a cyclic polydiorganosiloxane, and/or a substantially linear hydroxy terminated polydiorganosiloxane, causing the mixture to react in the presence of an acid or phosphazene base catalyst at a temperature of up to 180° C., and neutralising the reaction mixture.